Cisco WS-CBS3032-DEL Software Configuration Manual page 990

Configuring BGP
Before exchanging information with an external autonomous system, BGP ensures that networks in the
autonomous system can be reached by defining internal BGP peering among routers and by
redistributing BGP routing information to IGPs that run in the autonomous system, such as IGRP and
OSPF.
Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the
Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers
that have a TCP connection to each other are known as peers or neighbors. In
and B are BGP peers, as are Routers B and C and Routers C and D. The routing information is a series
of autonomous-system numbers that describe the full path to the destination network. BGP uses this
information to construct a loop-free map of autonomous systems.
The network has these characteristics:
• Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP
peers are directly connected and that the IBGP peers are not. As long as an IGP allows the two
neighbors to reach one another, IBGP peers do not have to be directly connected.
All BGP speakers within an autonomous system must establish a peer relationship. That is, the BGP
•
speakers within an autonomous system must have a logical full mesh. However, BGP4 provides
techniques to reduce the requirement for a logical full mesh: confederations and route reflectors.
Autonomous system AS 200 is a transit autonomous system for AS 100 and AS 300—that is, AS 200
•
transfers packets between AS 100 and AS 300.
BGP peers first exchange their full BGP routing tables and then send only incremental updates. BGP
peers also exchange keepalive messages (to ensure that the connection is up) and notification messages
(in response to errors or special conditions).
In BGP, each route consists of a network number, a list of autonomous systems that information has
passed through (the autonomous system path), and a list of other path attributes. The primary function
of a BGP system is to exchange network reachability information, including information about the list
of autonomous-system paths, with other BGP systems. This information determines autonomous-system
connectivity, to prune routing loops, and to enforce autonomous-system-level policy decisions.
A router or switch running Cisco IOS does not select or use an IBGP route unless it has a route available
to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is
disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the
"Configuring BGP Decision Attributes" section on page 39-56
BGP Version 4 supports classless interdomain routing (CIDR), so you can reduce the size of your routing
tables by creating aggregate routes, resulting in supernets. CIDR eliminates the network classes within
BGP and supports the advertising of IP prefixes.
These sections contain this configuration information:
Default BGP Configuration, page 39-49
•
Enabling BGP Routing, page 39-52
•
Managing Routing Policy Changes, page 39-55
•
Configuring BGP Decision Attributes, page 39-56
•
Configuring BGP Filtering with Route Maps, page 39-58
•
• Configuring BGP Filtering by Neighbor, page 39-59
• Configuring Prefix Lists for BGP Filtering, page 39-60
• Configuring BGP Community Filtering, page 39-61
Configuring BGP Neighbors and Peer Groups, page 39-63
•
Configuring Aggregate Addresses, page 39-65
•
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
39-48
Chapter 39 Configuring IP Unicast Routing
Figure 39-5, Routers A
for information about BGP attributes.
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